There has been known a hazard alarm system which comprises a central control unit and a plurality of alarm signal transmitting junction units connected to the former, from each of which extends one zone line having at least one sensor (heat sensor, smoke sensor, or the like) connected thereto and located at a place where any hazard must be monitored, wherein the central control unit calls the signal transmitting junction units in turn by polling, for instance requesting send-back of the signal from the sensor connected to the junction unit which has been called, and the central control unit judges the situation--normal situation, disorder of the device, or occurrence of a hazard such as fire, gas leak, etc. from the received signal and generates an alarm in accordance with the situation.
The signal-transmitting junction unit of such a hazard alarm system generally requires a priority determination circuit, encoder, etc. to produce signals to be sent to the central control unit. Also when the detection of disorder such as break of line is conducted intermittently, the junction unit effects the detection when it is called, or otherwise it has to memorize the detected results until it is called next time. Therefore, the circuit structure thereof is inevitably very complicated. A signal transmitting junction unit of a simple structure is not known until today.